Method of growing vertebrate skin in vitro

ABSTRACT

A method of growing complete vertebrate skin in vitro, which comprises obtaining a segment of vertebrate skin, positioning the skin segment in an artificial cell-growth medium containing sufficient nutrients to maintain growth of cells of the skin, and subjecting the skin segment to stretching forces while the skin segment is in the medium. Skin produced by the method and an apparatus for carrying out the method are also part of the present invention.

TECHNICAL FIELD

This invention is in the field of cell and tissue culture and isparticularly directed to the production of vertebrate skin having both adermal layer and an epidermal layer.

BACKGROUND

Vertebrate skin is a complex organ comprising two principal layers, anouter epidermal layer and a dermal layer lying under the epidermallayer. In order for skin to retain its normal appearance and to functionfully in a normal manner, both layers of the skin need to be present.

Skin grafting is an essential component of reconstructive surgery afterburns, trauma, tumor excision, and correction of congenital anomalies.There are approximately 1 million burns per year in the U.S. alone,which result in about 100,000 admissions to burn units, about 1/3 ofwhich require skin grafting. Skin grafting in reconstructive surgery isoften required to alleviate deformity. The best possible skin availablefor grafting would be skin from the same patient taken from a donor siteelsewhere on the body (referred to as an autograft). Suitable skin graftdonor sites, however, are limited not only by body surface area, but canalso be affected by previous graft harvest or trauma. There are times,therefore, when donor skin is limited and the amount of skin requiredfor grafting is quite large, so that sufficient autografts are notavailable. Because of the importance of the skin in preventinginfection, either the donor skin must be used to cover a larger areathan it originally covered or some suitable replacement material must beused.

Currently, several techniques are used to enhance the amount of donorarea skin. Meshing of donor skin (slitting the skin to form anexpandable mesh pattern) is used to increase the total area of graft.However, meshing is only minimally able to increase graft size while itsignificantly detracts from the appearance of grafts, making themunacceptable for reconstruction on the face, and far from ideal on thehands, arms, and neck. In patients suffering from large burns withlimited donor skin sites, cadaver allografts are commonly used fortemporary skin coverage, but ultimately such allografts are rejected,and a permanent autograft is required. In addition, allografts also posea risk of infection of the recipient by viruses or other disease-causingorganisms present in the donor, such as infection by humanimmunodeficiency virus or hepatitis virus.

To aid in the grafting of patients with limited donor areas, culturedepithelial cells derived from the patient being treated have beenutilized in many grafting applications. In general, the cells are usedin the form of a monolayer of epithelial cells grown on a culturemedium. Preparation of such cultures requires many weeks or months, andthe product is quite difficult to handle because of its fragility, evenwhen multiple epidermal cell layers are used to form a multi-layer skinsubstitute.

Harvesting of multiple skin grafts from the same donor site is oftenused, but such harvesting requires weeks to months between proceduresfor new skin to grow on the donor site. It is also a very traumatictechnique, since multiple painful operations must be undertaken.

Tissue expansion techniques, which are in vivo techniques, have beenused in plastic surgery for over a decade and can be helpful inincreasing the area of donor tissue. By placing an expandersubcutaneously and frequently injecting it with saline, skin can beexpanded and its surface area increased. This allows reconstruction withlocal skin after expansion of an adjacent tissue bed. Expanders are notideal, however, because they require multiple procedures. When localtissue is of poor quality, as might be the case in a patient who hasundergone multiple reconstructions or irradiation or has been burned,expanders are not a viable option. For examples of the generalapplication of tissue expansion, see, for example, Argenta, "Controlledtissue expansion in reconstructive tissue," Brit. J. Plas. Surg.,37:520-529 (1984), and Argenta et at., "The Use of Tissue Expansion inHead and Neck Reconstruction," Ann. Plast. Surg., 11:31-37 (1983).

Additional background information in the general field of skin graftingis available in the scientific literature. A number of exemplarypublications are cited below:

Kirsner et al, "The Biology of Skin Grafts," Arch. Dermatol.,129:481-483 (1993).

Gallico, "Biologic Skin Substitutes," Clinics in Plastic Surgery,17:519-526 (1990).

Nanchahal and Ward, "New grafts for old? A review of alternatives toautologous skin," Brit. J. Plas. Surg., 45:354-363 (1992).

Carney, "Generation of autograft; the state of the art," Burns.12:231-235 (1986).

Greenwald et at., "Full-Thickness Skin Wound Explants in TissueCultures: A Mechanical Evaluation of Healing," Plastic andReconstructive Surgery, 90:289-294 (1992).

Arons et at., "The surgical applications and implications of culturedhuman epidermis: A comprehensive review," Surgery, 111:4-11 (1992).

In view of the shortcomings of the prior art recited above, a techniquethat provides a large surface area of normal skin from a small donorskin segment is therefore needed. Desirably, the skin should be anautograft and should be available in as short a time period as possible.Such a technique would be of great benefit to a reconstructive surgerypatient because it would limit the number of surgical proceduresrequired on a patient. It will also potentially increase survival byclosing wounds more promptly in a patient who requires a large amount ofskin grafting.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain aspects of the invention will be better understood by referenceto the drawing that form part of the specification, wherein:

FIG. 1 is a vertical cross-sectional view of a first embodiment of anapparatus of the invention; and

FIG. 2 is a vertical cross-sectional view of a second embodiment of anapparatus of the invention.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a method ofproducing a normal skin autograft.

It is a further object of the invention to provide a normal skinautograft having a larger surface area than the area of the donor graftsite.

It is yet another object of the invention to provide a normal skinautograft in as sort a time period as possible.

It is a still further object of the invention to provide hair- orfur-bearing skin that can be produced in large quantities in vitro.

These and other objects of the invention have been accomplished byproviding a method of growing vertebrate skin in vitro, which comprisesobtaining a segment of vertebrate skin containing an epidermal layer andat least part of the normal dermal layer, positioning said skin segmentin an artificial cell-growth medium containing sufficient nutrients tomaintain growth of cells of said skin, and subjecting said skin segmentto stretching forces while said skin segment is in said medium. Skinproduced by this technique is also an object of the invention.

In various embodiments of the invention, the stretching forces aredynamic or static, orthogonal or radial, or constant or varying. Theskin is preferably human for use in human recipients, but veterinary useof the new skin is also encompassed by the invention. Other embodimentsof the invention are set forth below in detail.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The present invention combines a number of previously known techniquesin a novel manner to achieve results not previously obtained, namelynormal autograft skin segments (containing both a dermal layer and anepidermal layer) that are larger than the autograft donor site and thatare prepared in vitro. The method of the invention makes availabledurable autologous skin greatly increased in size available for transferto either nearby or distant locations. By using stretching forces onskin in vitro during tissue culture, a goal of maximizing the potentialavailability of skin while decreasing patient morbidity associated withskin grafting can be achieved. In addition, problems which prior to nowexist using allografts and cultured epithelial cells would not beencountered. Potentially this technique could save costs by minimizingthe number of surgical procedures a patient would require while alsodecreasing the risk associated with multiple anesthetics and surgicalprocedures sometimes needed in cases of complex reconstruction.

There are a number of technologies ancillary to the present inventionthat are well developed and that thus will not be described here ingreat detail, such as methods for excision of the original skin segment,production of culture media for the growth and maintenance of intactskin, and grafting techniques for the attachment of the graft to thehost. Such methods and materials are exemplified here, and referencesare given to scientific publications, where appropriate, so that theinvention can readily be practiced. It will be recognized, however, byone of ordinary skill in the art, that many variations of theseancillary methods and materials exist and that the invention is notlimited to the specific examples provided here.

In general, the method of the invention provides a method of growingcomplete vertebrate skin in vitro, which comprises obtaining a segmentof vertebrate skin, positioning the skin segment in an artificialcell-growth medium containing sufficient nutrients to maintain growth ofcells of the skin segment, and subjecting the skin segment to stretchingforces while the skin segment is in the medium. Here "in the medium"means that at least the dermal side of the skin is in contact with themedium, since the epidermal layer of skin is normally exposed to air.However, the skin segment can be completely sumberged in the nutrientmedium, if desired. When skin is present in a growth medium while beingsubjected to stretching forces, cell division and growth occur so as toproduce normal, full-thickness skin having both a dermal layer and anepidermal layer. The skin produced in this manner is particularlysuitable for grafting in place of complete skin loss (i.e., loss of boththe dermal layer and the epidermal layer) and for other uses describedherein.

In the following description of various embodiments of the invention,the invention is described in most cases as being practiced with a humanskin autograft. However, the invention is not so limited and can be usedfor both allografts (within the same species, but with the donor andrecipient being different individuals) and xenografts (donor andrecipient from different species). Additionally, the invention is notlimited to preparation of human skin, since it can be advantageouslypracticed to produce large amounts of normal skin of variousvertebrates, either for veterinary use as autografts or allografts, orfor use in the production of xenografts (which would normally requiresuppression of the immune system of the recipient when the product skinis used as a graft), since even the temporary protection againstinfection and sepsis provided by a xenograft may save the life of therecipient organism. For veterinary or xenograft use, the donor skin canbe obtained from any vertebrate, preferably one related as closely aspossible to the recipient. Non-exclusive examples include skin fromhumans, other primates, cattle and other domesticated bovines, pigs,hogs, cats, dogs, sheep, goats, birds, and reptiles. The method can alsobe used in the fur industry to produce artificial furs that do notrequire the sacrifice of animals, such as animals that are difficult toraise domestically or that are endangered, including sea otters, seals,tigers, and the like. Production of hair-bearing human skin for use intreating baldness is also a part of the invention. For the production ofstretched skin with hair or fur, full-thickness skin is used, assplit-thickness skin lacks hair follicles.

A segment of vertebrate skin (graft donor segment) is obtained by any ofthe techniques normally available for this purpose, usually surgicalexcision (for full-thickness skin) or use of a dermatome (forsplit-thickness skin). If a dermatome (i.e., any plane-like device forremoving skin from a subject) is used, the thickness of the layer shouldbe selected to ensure that at least some of the dermal layer is present.This thickness will vary from species to species and even from locationto location on the body of an individual. A typical setting for adermatome used to prepare split-thickness human skin is about 12/1000thof an inch (about 0.3 mm). The skin segment is obtained so that bothdermal and epidermal layers are present in the detached segment. Thedermal layer can be either complete (full-thickness skin) or incomplete(split-thickness skin), but some of the dermal layer should be present.

Full-thickness skin segments are generally obtained by surgicalexcision, while split-layer skin segments are obtained by a dermatome.Both of these techniques, as well as other general techniques in thefield of skin grafting, as described in Chapter 1 (pp. 1-90) of Grabband Smith, Plastic Surgery, Little Brown & Company, Boston, Mass., USA,4th Ed. (1991), James W. Smith and Sherrel J. Aston, eds. The detachedskin is normally transferred directly to a culture medium, and in mostcases in not allowed to dry out before being positioned in the medium.The shape of the detached skin segment is not material to the practiceof the invention, but certain shapes will be better suited to individualspecific apparatus variations described here.

The size of the donor skin segment is generally selected for theconvenience of use with the apparatus in which it will be stretched andmay also vary depending on the availability of donor skin tissue of thesame type as that being replaced. Typical human skin segments are from1×1 cm to 10×30 cm but can vary significantly depending on theavailability of donor skin. There is generally no impact of graft sizeon the method of the invention, so that surgical and other proceduresgenerally are more important in determining tissue size. For ease ofhandling in surgical skin grafting, segments ranging in size from 5×5 cmto 15×15 cm are preferred.

Preparation and use of artificial cell-growth media containingsufficient nutrients to maintain growth of cells of a skin segment arewell established techniques and need not be described here in detail.Such media are also referred to as nutrient media or tissue-culturemedia. Whether any given medium will be satisfactory (if not alreadyknown) can easily be determined experimentally using the procedures forskin growth set out in the examples below. Many such media arecommercially available, such as Dulbecco's modified Eagle's medium(DMEM) with 10% added fetal calf serum. Other suitable media includebasal medium (Eagle) with Hanks's BSS (85%) supplemented with calf serum(15%) and Ham's F12 medium (90%) supplemented with fetal bovine serum(10%). When serum is used to supplement an artificial medium, fetalserum is preferred, especially fetal serum from the same species as therecipient of the graft. When this is not possible or ethicallydesirable, the recipient's own serum can be used. For a number of mediathat can be used to grow skin tissue, see, for example, the mediaformulations section of any volume of the American Type CultureCollection publication entitled Catalogue of Cell Lines & Hybridomas(e.g., 5th edition, 1985, pages 265-273). This ATCC publication alsocontains information (in connection with specific skin-derived celllines) on which media are best for use with tissue or cell culturesderived from skin.

The apparatus in which tissue culture and stretching takes place canvary widely, being either simple or complex. An example of a simpleapparatus is a Petri (or similar) dish containing a tissue-culturemedium and having a set of clamps, wires, pulleys, and weights arrangedso that the clamps can be attached to a detached skin segment in themedium and subjected to forces applied to clamps by weights attached tothe clamps by the wires and suspended by the pulleys to reduce friction.A more complex apparatus could contain electric motors for supplyingforce to the clamps or for circulating the culture medium in theapparatus, sensors to measure forces and stretching distances,reservoirs for fresh and waste medium, controlled atmospheres, and thelike. Minimally, an apparatus of the invention will comprise a containerfor holding a tissue culture medium, at least two connectors for holdinga detached skin segment in the culture medium at at least two locationson the skin segment, the connectors being attached to the container, andmeans for supplying opposing forces via the attachment means to the skinsegment. The container may be an integral part of the apparatus or maybe a separate container that is retained by the apparatus at a specificlocation. In the later case the connectors are "attached" indirectly tothe container by being affixed to the frame of the apparatus that willhold the container.

An alternative apparatus can comprise a tubular fluid reservoir havingan open end, a clamp located at the open end of the reservoir, where theclamp is adapted to seal the skin segment over the open end to provide afluid-tight seal, and means for supplying hydrostatic pressure to afluid located in the reservoir. It should be recognized here that"tubular" does not require a circular cross section, as the word is usedhere. Examples of means for supplying hydrostatic pressure comprises (1)a nutrient reservoir fluidly connected to the tubular fluid reservoirand being located at a higher gravitational potential than the skinsegment when the apparatus is located in its normal operating positionor (2) a pump fluidly connected to the tubular reservoir. A pump is anymechanical device that moves fluid from one location to another andincludes a hydraulic piston. An example of a suitable clamp would be anannular member adapted to fit tightly against a flange on the open endof the tubular reservoir, with holes or grooves in the flange or annularmember (or both) adapted to contain screws, bolts, wing nuts, or thelike for fastening the annular member against the flange, with the skinsegment being located between them to provide a fluid-tight seal at theend of the tubular reservoir with the skin being attached over the endof the reservoir as in a drum. Since fleshly harvested skin isresilient, no additional seal is required, but a flexible sealing member(such as an O-ring) can be provided between, e.g., the flange andannular member, if desired.

The two embodiments described above are exemplified in FIGS. 1 and 2, inwhich the same reference numerals refer to corresponding features of thedifferent embodiments.

FIG. 1 shows a first embodiment of an apparatus of the invention inwhich nutrient reservoir 10 is supported by legs 20. Skin segment 30 isheld in place by clamps 40 attached via wires 60 than run over pulleys50 to freely suspended weights 70. Nutrient medium 80 is present inreservoir 10 in an amount sufficient to cover skin segment 30.

FIG. 2 shows a second embodiment of an apparatus of the invention inwhich tubular reservoir 10 contains nutrient medium 80. Skin segment 30is held in place over the end of reservoir 10 by clamp 40, which in thisembodiment is an annular member 42 having grooves 44 that match with andengage pivoted wing bolts 12 that are attached to flange 14 at the endof reservoir 10. Hydraulic pressure is supplied by pump 70 that isfluidly connected via piping 72 to reservoir 10.

The reader is directed to any of the patents or other publications inthe field of tissue culture for details of such apparatuses related toaspects of general tissue culture.

A key aspect of the invention is subjecting the detached skin segment tostretching forces while the skin segment is in the tissue culturemedium. Here "stretching forces" means a force or forces applied to theskin segment in one or more direction parallel to the plane of the skinsurface. Because of the physical nature of stretching forces, at leasttwo opposed forces are applied to a skin segment (because of Newton'sfamiliar law of equal and opposite forces). If only two opposed forcesare present the stretching is along a line coaxial with the stretchingforces, subject to some additional stretching along adjacent regions ofthe detached skin segment. This is the simplest stretching situation,but is not particularly desired because of the resulting skindeformation. Additional forces can applied to provide for more regularstretching of the skin. Parallel opposed forces (such as would beapplied by two broad, rigid clamps attached to opposite ends of adetached skin segment) lead to stretching along a single dimension ofthe skin. Non-parallel multiple stretching forces (e.g., radial outwardfrom a central point or orthogonally in the plane of the skin) result instretching in both of the two dimensions of a detached skin segment(i.e., the two dimensions parallel to the plane of the skin surface).

Forces can be applied to the skin that are not entirely parallel to theskin surface; however, some portion of the force must be parallel to theskin surface for stretching to take place. For example, a convex solidsurface or a fluid forced against the face of a detached skin segmentwhose edges are fixed will cause the skin segment to be subjected toforces both orthogonal and parallel to the surface of the skin; suchstretching comes within the scope of the present invention.

While the detached skin segment is being stretched, the ends of thesegment are held in place in the tissue culture by some physicalapparatus. Any apparatus that can be used to hold the ends in place canbe used. An attachment apparatus is need for each point to which a forcewill be applied. Typical attachment apparatuses include clamps, hooks,sutures, and glue. A clamp can be narrow (e.g., less than 1/10 thelength of the edge being held) or broad (up to or greater than the widthof the edge, and generally considered broad when greater in width than1/2 the width of the edge). If opposed broad clamps are used, stretchingbetween the ends of the clamp will generally be restricted if anorthogonal stretching force is also present on the skin. For maximumstretching efficiency, multiple attachment points capable of moving awayfrom each other during the stretching process are preferred. Forexample, multiple small hooks or clamps attached in a generally circularmanner to a circular detached skin segment and subjected to forcesapplied radially outward from the center of the segment automaticallymove away from each other as stretching proceeds, thus supplyingstretching forces along the tangents of the circle as well as along itsradii.

The forces themselves can be supplied by any means for supplying force,such as a weight, spring, or motor. The forces can be either static ordynamic. Here a static force is one which is applied between twoattachment points that do not move further apart from each other as cellgrowth and division occurs to reduce over time the force between theattachment points. For example, two clamps can be attached to oppositeends of a detached skin segment, with one (or both) of the clamps beingattached to a screw that the distance between the clamps can be varied.Turning the screw to move the clamped ends away from each other producesan initial force on the skin segment, but this force decreases as cellsin the skin grow and divide. A dynamic force, on the other hand, is oneprovided between two attachment points that are capable of movement sothat a constant force can be maintained. For example, two clamps can beattached to weights that are suspended via a pulley system from oppositeends of a detached skin segment. The force on the skin segment in suchan apparatus remains constant as the skin grows and divides.

The amount of the force applied to the skin is minimally that requiredto cause the skin to stretch and will not exceed the amount required tocause the skin to rupture. Since the strength of different skin segmentsobtained from the same donor vary, the forces are best determinedempirically by the amount of skin stretch that is obtained. In manycases, the actual forces will never even be measured or known, such asin a screw-based apparatus. A typical stretched skin segment has an areaafter being subjected to stretching forces (over an appropriate lengthof time) that is at least twice that of the skin segment prior to beingsubjected to the stretching forces. For human skin, stretching of atleast 2% per day is desired, preferably at least 5%, more preferably atleast 10%. Non-human skin can be either tougher or less tough (here"tough" refers to resistance to stretching) than human skin and thus maybe stretched correspondingly less or more than these amounts. In generalskin can be stretched until rupture or cell death induced by the tensionof stretching, which can readily be followed by histologicalexamination. In some cases it may be desirable to keep stretching under15% per day to avoid cell death, in other cases under 12%. However, themaximum sustainable stretch rate is best determined empirically, usingthese numbers as initial guidelines. When skin is initially placed inthe nutrient medium, it should be stretched back to in original in vivosize before actual stretching is measured, since skin removed from abody generally shrinks to about one-half of its original dimensions.When hair- or fur-bearing skin is being grown, an additional factor tobe considered is selecting a stretch rate is the rate of generation ofnew hair follicles, which will occur along with other cell growth anddivision in full-thickness skin. The rate of stretching can be selectedin such cases to provide the desired fur (hair) density, rather thansimply selecting for the maximum sustainable stretch rate.

After the stretched skin segment has regrown its original thickness, theskin segment can be used as a graft or be divided into further segmentsso that one or more of the further segments is subjected again to themethod of the invention. As long as cell growth continues, new normalskin can be produced from parts of the original donor graft. Skin havinga surface are at least two times that of the original donated skinsegment can be provided in one stretching operation (which may last overseveral days), usually at least four times, and often at least eightthat of the original. Since the stretched skin can then be divided andand the resulting segments re-stretched, exponential production of skinis possible.

Once a stretched skin graft product has been prepared, it is used inskin grafting in the same manner as an unstretched graft donor skinsegment. Fur-bearing skin can be used in the normal manner of usinganimal pelts for making fur garments.

The invention now being generally described, the same will be betterunderstood by reference to the following detailed examples of theinvention, which are provided here for illustration only and are not tobe considered limiting of the invention unless so specified.

EXAMPLE MATERIALS AND METHODS

Sprague-Dawley rats are used for harvesting and placement of the treatedskin grafts. All surgical procedures are carried out under generalanesthesia following standard laboratory techniques and animal careguidelines. Each rat is anesthetized with pentobarbital 50 mg/kgintraperitoneally. A full thickness ventral skin graft is then harvestedusing standard aseptic technique, and the wound is closed withabsorbable suture. One half of these grafts are thinned to splitthickness grafts using the Reese dermatome, and one half of the graftsare treated as full thickness grafts. The animals are observed untilawake and ambulating and are then returned to their housing facility.

Skin grafts are measured in size before and after harvesting and arethen stretched in vitro using different types of skin stretchingdevices. Stretching generally utilizes a daily increase in the expansiondistance. All grafts are placed in Dulbecco's modified Eagle's medium(DMEM) optionally containing 10% fetal bovine serum and placed onsterile petri dishes during expansion. In some experiments 10% fetalcalf serum is added to the mixture, as it provides many nutrientsdesirable for maximum rate of cell growth. To maintain a constantenvironment for skin graft support during stretching, the grafts arestored in an incubator at 37° C. with an atmosphere containing 5% carbondioxide. The grafts are observed and stretched daily, and the growthmedium is replaced every other day.

The skin graft expansion are divided into groups. One group of full andsplit thickness grafts is stretched initially to 2× normal size. Asecond group is stretched to 4× normal, and a third group to 8× normalsize. Some grafts are stretched further.

Initial studies are carried out at 37° C. (human core temperature).However, lower temperatures can be used to slow metabolism of the skingraft and increase the viability, "stretchability," and growthpotential.

After stretching of the skin grafts and recovery of the skin to itsoriginal state, the animals are again anesthetized with pentobarbital.The skin grafts are attached to the dorsal surface of the animal after asurgical defect in the skin is created by simple skin excision. The sizeof the graft is measured and recorded at the time of grafting. Theanimals are monitored daily until postoperative day 14 to examine skingraft take and contracture of the graft. At postoperative day 14 theanimals are sacrificed and paper patterns are made of the surviving skingraft area and its size. The size of the surviving area is compared tothat removed at the initial procedure and that grafted after in-vitrostretching. A group of non-expanded grafts that are placed in DMEM areused as controls. The experimental animals are compared with thecontrols using standard statistical methods to look for any deviation ofthe means. The number of animals used is selected to produce astatistically significant result without excess animal sacrifice. Intypical studies done previously, between 10 and 30 animals have beenused for statistical significance to be evident. The standard deviationin the control group will vary moderately because of differentialsurvival of skin grafts even by standard techniques. A sample size of 20experimental rats will generally provide a sample large enough to ensurestatistical analysis.

RESULTS

Results obtained to date verify the premise of the invention. Ventralrat full-thickness skin grafts were harvested and split with a Reesedermatome to provide split-thickness skin segments. Full-thickness skingraft segments were used as well. The rat skin grafts of approximately 4cm square (prior to harvest) were stretched in vitro by wrapping thegraft around a tissue expander (essentially an expandable balloon), andthe graft sutured to itself around the expander using absorbable suture.The expander with the graft around it was placed in a bath of tissueculture medium (DMEM) and placed in an incubator at 37° C. Every day theexpander was removed from the bath, and water was injected into theexpander to cause further expansion. The experiment was stopped when thesutures tore through the graft. However, the expanded skin showed markedincrease in size with daily inflation of the expanders, so that by theend of one week, almost a 2-fold increase in surface area was noted.This success rate was repeated several times.

All publications and patent applications mentioned in this specificationare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

The invention now being fully described, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade thereto without departing from the spirit or scope of the appendedclaims.

What is claimed is:
 1. A method of growing complete vertebrate skin invitro, which comprises:obtaining a detached segment of vertebrate skincomprising an epidermal layer and a dermal layer and having a firstsurface area; positioning said skin segment in an artificial cell-growthmedium containing sufficient nutrients to maintain growth of cells ofsaid skin; subjecting said skin segment to stretching forces while saidskin segment is in said medium; and collecting said skin after said skinhas grown to have an expanded surface area greater than said firstsurface area and to have dermal and epidermal layers over said expandedsurface area.
 2. The method of claim 1, wherein said stretching forcesstretch said skin segment in one dimension.
 3. The method of claim 1,wherein said stretching forces stretch said skin segment in twodimension.
 4. The method of claim 1, wherein said skin segment has anarea after being subjected to said stretching forces at least twice thatof said skin segment prior to being subjected to said stretching forces.5. The method of claim 1, wherein said skin segment is subjected tostatic stretching forces.
 6. The method of claim 5, wherein after saidskin segment has regrown full thickness, said skin segment is dividedinto further segments, and at least one of said further segments issubjected to said method.
 7. The method of claim 1, wherein said skinsegment is subjected to dynamic stretching forces.
 8. The method ofclaim 1, wherein said skin segment is mammalian skin.
 9. The method ofclaim 8, wherein said skin segment is human skin.
 10. A detached skinsegment having a dermal layer and an epidermal layer and being producedby the method of claim 1.